A field emission is a phenomenon in which electrons are emitted into a vacuum by means of electric-field concentration. As an electron emitter for generating this field emission, for example, a carbon nanotube has attracted attention. Since this carbon nanotube is extremely narrow and has a high-aspect ratio, the carbon nanotube has a superior field emission characteristic. Hence, the carbon nanotube is thought to be able to produce a field-electron emission element. Accordingly, it has been considered that the carbon nanotube is applied to various field emission devices such as the electron tube and the illuminating system.
The field emission characteristic (IV characteristic) is shown by a curved line representing a relation between a voltage V and a field emission current (emitted current) I when an electric field is emitted from a cold cathode by applying the voltage V between the cold cathode and an anode. This field emission characteristic (IV characteristic) is characterized by a voltage value (threshold value) for starting the field emission, and gradient and shape of the curved line.
As a concrete example of the field emission device, a cold cathode fluorescent lamp can be cited in which the above-mentioned cold cathode is disposed to face an anode provided with a fluorescent material. In the cold cathode fluorescent lamp, electrons are made to be emitted by field emission from the cold cathode by generating a voltage (anode-to-cathode voltage) between the cold cathode and the anode, and these emitted electrons are accelerated and collided to the fluorescent material so as to excite the fluorescent material to become luminous. This luminescence (light emission) of fluorescent material needs a predetermined amount of electron emission. The current-voltage (I-V) characteristic curve having a vertical axis representing an emission current corresponding to an amount of electron emission and a lateral axis representing the anode-to-cathode voltage means an electron emitting performance of the cold cathode. In the case of carbon nanotube, the gradient of the above-mentioned I-V characteristic curve starts to rise moderately. Hence, in the case of carbon nanotube, a voltage V necessary to obtain the emission current value for starting the luminescence (light emission) of fluorescent material is relatively high.
However, since a value of the applied voltage V for obtaining a desired emission current is large; a characteristic of the carbon nanotube itself is changed (deteriorates), and also, a voltage necessary to obtain a certain current becomes high. Therefore, for example, there are a problem that a power-supply facility for this high voltage is required and a problem that a production of the cold cathode fluorescent lamp is affected. Accordingly, it has been awaited to realize a carbon film for cold cathode which provides an I-V characteristic that can obtain an emission current capable of causing the fluorescent material to start to become luminous with a relatively low applied voltage V.
In recent years, a carbon film structure which is formed by dispersing a plurality of acute shapes (i.e., countless number of acute shapes are dispersed) on a surface of substrate, has been developed by the inventor of the present application, etc., instead of the carbon nanotube or the like. Each of the plurality of acute shapes is formed by piling graphene sheets in a multilayer manner to have an inside hollow portion, and has a radius which becomes smaller as its tip approaches. That is, this carbon film structure is constructed by forming a plurality of carbon film aggregation units on the substrate. Each of these carbon film aggregation units includes a stem-shaped carbon film and a branch-shaped carbon film group. This branch-shaped carbon film group is formed to surround the stem-shaped carbon film from the middle of the stem-shaped carbon film to a lower portion of the stem-shaped carbon film. The stem-shaped carbon film is formed with the inside hollow portion by the multi-piled graphene sheets, and is formed in the acute shape reducing its radius toward its tip end (for example, such a structure is disclosed by Patent Documents 1 and 2). An emitter having such a carbon film structure can obtain a desired emission current with a lower applied-voltage as compared with the carbon nanotube and the like, because of the existence of the acicular acute shapes whose radius is reduced toward its tip. Therefore, the emitter having the above-mentioned carbon film structure is thought to be able to provide a field emission device having a superior performance in I-V characteristic.
FIG. 7 is a schematic view showing a film-forming apparatus that uses a plasma CVD method (direct-current plasma film-forming method), as one example of a system for forming the carbon film structure. As shown in FIG. 7, a vacuum film-forming chamber 1 is equipped with a gas introducing system 2 (for example, an introducing system for a gas mixture of a hydrogen gas and a gas containing carbon (e.g., methane gas)) and an evacuating system 3. Inside the vacuum film-forming chamber 1, a cathode 4 (an electrode including an insulation cooling plate 4a for controlling a cathode temperature) and an anode 5 are disposed to face each other. A reference sign 6 denotes a direct-current power source. A negative pole side of the direct-current power source 6 is connected with the cathode 4. A positive pole side of the direct-current power source 6 and the anode 5 are respectively grounded.
As to such a film-forming apparatus, at first, the evacuating system 3 evacuates the vacuum film-forming chamber 1. Then, the gas introducing system 2 introduces the gas (hydrogen gas) and gradually controls a pressure of the vacuum film-forming chamber 1 (for example, to about 30 torr). An electric current is maintained at a desired level (for example, about 2.5 A). Thereby, oxides on the substrate 7 are eliminated.
Next, the gas introducing system 2 introduces the gas mixture into the vacuum film-forming chamber 1 so as to gradually increase the internal pressure of the vacuum film-forming chamber 1, and then, maintains the internal pressure of vacuum film-forming chamber 1 (for example, at about 75 torr). The current of direct-current power source 6 is gradually increased and maintained (for example, at about 6 A).
Accordingly, the temperature of substrate 7 becomes equal to a predetermined temperature (for example, about 900° C. to 1150° C.) by plasma 8 generated on the substrate 7. Thereby, the gas containing carbon which is included in the above-mentioned gas mixture is decomposed so that the carbon film structure (reference sign 10 in an after-mentioned FIG. 8) is formed on a surface of the substrate 7. In the case that the carbon film structure is formed in this manner, a mask (not shown) may be suitably used for the substrate 7.
FIG. 8 is a schematic explanatory view in the case that an electron emitter including the carbon film structure formed as mentioned above is used as the cold cathode. As shown in FIG. 8, at first, an electrode surface (upper surface in FIG. 8) of cold cathode 9 which is located on the side of carbon film structure 10 and an electrode surface (lower surface in FIG. 8) of anode 11 are disposed to face each other (i.e., the respective electrode surfaces are arranged parallel to each other). Then, when a direct-current power source 12 applies a constant voltage between the both electrodes, tunnel electrons shown by a Fowler-Nordheim formula are emitted from the cold cathode 9 into the anode 11, by a strong electric field formed at the carbon film structure 10 (in particular, at the tip of each acute shape). An electron emission characteristic in this case is shown by FIG. 9. It is preferable that an emission direction of electron is perpendicular to the electrode surface of cold cathode 9.
However, as to the electron emitter including the above-mentioned carbon film structure, a growth direction or a shape (size, thickness and the like) of each acute shape of this carbon film structure is difficult to equalize. In particular, in the case that the carbon film structure is formed by using the mask for the substrate, a relatively thick and dense portion of carbon film structure is formed around the mask (for example in the case of FIG. 8, at an outer circumferential edge portion of the carbon film structure 10).
Therefore, if a field emission device equipped with the cold cathode simply including the above-mentioned carbon film structure is used, the emission direction of electron deviates from the direction perpendicular to the cold cathode surface (i.e., electrons are emitted in various directions so as to be dispersed as shown by broken lines of FIG. 8). Thereby, a region of electron flow between the cold cathode and the anode is expanded. Hence, an electron spot in the anode becomes large and uneven (for example in the case of FIG. 8, the electron spot of the anode 11 is larger than an area of the electrode surface of cold cathode 9 and is inhomogeneous as compared to the electrode surface of cold cathode 9). Accordingly, it is difficult to obtain a high current density, so that a large and stable current is not obtained.
Moreover, in the case of the above-mentioned relatively thick and dense carbon film structure, a localized electric-field concentration is easy to cause. Hence, equipotential surfaces protrude at this region of localized electric-field concentration. Thereby, a large quantity of electrons are emitted to cause a current degradation due to thermal degradation or to cause an electric-discharge phenomenon due to charge-up and subsequent insulation breakdown to structural members existing around the cold cathode.
When trying to attain a desired function (for example, a function as electron-beam source) by applying the above-explained emitter to a field emission device, large-scaled power source and various types of equipments and the like are necessary. Thus, it has been difficult to obtain a practical-level product (for example, a compact and low-cost product).